Method and apparatus for maintaining cell wall integrity using a high yield strength outer sleeve

ABSTRACT

A method and apparatus is provided in which a pre-formed sleeve or pre-formed secondary can comprised of one or more layers of a high yield strength material is positioned around the pre-formed battery case, the pre-formed sleeve/secondary can inhibiting the flow of hot, pressurized gas from within the battery through perforations formed in the battery casing during a thermal runaway event.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/504,712, filed Jul. 17, 2009, now U.S. Pat. No. 7,749,647,issued Jul. 6, 2010, the disclosure of which is incorporated herein byreference for any and all purposes.

FIELD OF THE INVENTION

The present invention relates generally to batteries, and moreparticularly, to a means for maintaining cell wall integrity duringthermal runaway.

BACKGROUND OF THE INVENTION

Batteries can be broadly classified into primary and secondarybatteries. Primary batteries, also referred to as disposable batteries,are intended to be used until depleted, after which they are simplyreplaced with one or more new batteries. Secondary batteries, morecommonly referred to as rechargeable batteries, are capable of beingrepeatedly recharged and reused, therefore offering economic,environmental and ease-of-use benefits compared to a disposable battery.

Although rechargeable batteries offer a number of advantages overdisposable batteries, this type of battery is not without its drawbacks.In general, most of the disadvantages associated with rechargeablebatteries are due to the battery chemistries employed, as thesechemistries tend to be less stable than those used in primary cells. Dueto these relatively unstable chemistries, secondary cells often requirespecial handling during fabrication. Additionally, secondary cells suchas lithium-ion cells tend to be more prone to thermal runaway thanprimary cells, thermal runaway occurring when the internal reaction rateincreases to the point that more heat is being generated than can bewithdrawn, leading to a further increase in both reaction rate and heatgeneration. Eventually the amount of generated heat is great enough tolead to the combustion of the battery as well as materials in proximityto the battery. Thermal runaway may be initiated by a short circuitwithin the cell, improper cell use, physical abuse, manufacturingdefects, or exposure of the cell to extreme external temperatures.

Thermal runaway is of major concern since a single incident can lead tosignificant property damage and, in some circumstances, bodily harm orloss of life. When a battery undergoes thermal runaway, it typicallyemits a large quantity of smoke, jets of flaming liquid electrolyte, andsufficient heat to lead to the combustion and destruction of materialsin close proximity to the cell. If the cell undergoing thermal runawayis surrounded by one or more additional cells as is typical in a batterypack, then a single thermal runaway event can quickly lead to thethermal runaway of multiple cells which, in turn, can lead to much moreextensive collateral damage. Regardless of whether a single cell ormultiple cells are undergoing this phenomenon, if the initial fire isnot extinguished immediately, subsequent fires may be caused thatdramatically expand the degree of property damage. For example, thethermal runaway of a battery within an unattended laptop will likelyresult in not only the destruction of the laptop, but also at leastpartial destruction of its surroundings, e.g., home, office, car,laboratory, etc. If the laptop is on-board an aircraft, for examplewithin the cargo hold or a luggage compartment, the ensuing smoke andfire may lead to an emergency landing or, under more dire conditions, acrash landing. Similarly, the thermal runaway of one or more batterieswithin the battery pack of a hybrid or electric vehicle may destroy notonly the car, but may lead to a car wreck if the car is being driven, orthe destruction of its surroundings if the car is parked.

One approach to overcoming this problem is by reducing the risk ofthermal runaway. For example, to prevent batteries from being shortedout during storage and/or handling, precautions can be taken to ensurethat batteries are properly stored, for example by insulating thebattery terminals and using specifically designed battery storagecontainers. Another approach to overcoming the thermal runaway problemis to develop new cell chemistries and/or modify existing cellchemistries. For example, research is currently underway to developcomposite cathodes that are more tolerant of high charging potentials.Research is also underway to develop electrolyte additives that formmore stable passivation layers on the electrodes. Although this researchmay lead to improved cell chemistries and cell designs, currently thisresearch is only expected to reduce, not eliminate, the possibility ofthermal runaway.

FIG. 1 is a simplified cross-sectional view of a conventional battery100, for example a lithium ion battery utilizing the 18650 form-factor.Battery 100 includes a cylindrical case 101, an electrode assembly 103,and a cap assembly 105. Case 101 is typically made of a metal, such asnickel-plated steel, that has been selected such that it will not reactwith the battery materials, e.g., the electrolyte, electrode assembly,etc. Typically cell casing 101 is fabricated in such a way that thebottom surface 102 is integrated into the case, resulting in a seamlesslower cell casing. The open end of cell case 101 is sealed by a capassembly 105, assembly 105 including a battery terminal 107, e.g., thepositive terminal, and an insulator 109, insulator 109 preventingterminal 107 from making electrical contact with case 101. Although notshown, a typical cap assembly will also include an internal positivetemperature coefficient (PTC) current limiting device, a currentinterrupt device (CID), and a venting mechanism, the venting mechanismdesigned to rupture at high pressures and provide a pathway for cellcontents to escape. Additionally, cap assembly 105 may contain otherseals and elements depending upon the selected design/configuration.

Electrode assembly 103 is comprised of an anode sheet, a cathode sheetand an interposed separator, wound together in a spiral pattern oftenreferred to as a jellyroll'. An anode electrode tab 111 connects theanode electrode of the wound electrode assembly to the negative terminalwhile a cathode tab 113 connects the cathode electrode of the woundelectrode assembly to the positive terminal. In the illustratedembodiment, the negative terminal is case 101 and the positive terminalis terminal 107. In most configurations, battery 100 also includes apair of insulators 115/117. Case 101 includes a crimped portion 119 thatis designed to help hold the internal elements, e.g., seals, electrodeassembly, etc., in place.

In a conventional cell, such as the cell shown in FIG. 1, a variety ofdifferent abusive operating/charging conditions and/or manufacturingdefects may cause the cell to enter into thermal runaway, where theamount of internally generated heat is greater than that which can beeffectively withdrawn. As a result, a large amount of thermal energy israpidly released, heating the entire cell up to a temperature of 900° C.or more and causing the formation of localized hot spots where thetemperature may exceed 1500° C. Accompanying this energy release is therelease of gas, causing the gas pressure within the cell to increase.

To combat the effects of thermal runaway, a conventional cell willtypically include a venting element within the cap assembly. The purposeof the venting element is to release, in a somewhat controlled fashion,the gas generated during the thermal runaway event, thereby preventingthe internal gas pressure of the cell from exceeding its predeterminedoperating range.

While the venting element of a cell may prevent excessive internalpressure, this element may have little effect on the thermal aspects ofa thermal runaway event. For example, if a local hot spot occurs in cell100 at a location 121, the thermal energy released at this spot may besufficient to heat the adjacent area 123 of the single layer casing wall101 to above its melting point. Even if the temperature of area 123 isnot increased beyond its melting point, the temperature of area 123 inconcert with the increased internal cell pressure may quickly lead tothe casing wall being perforated at this location. Once perforated, theelevated internal cell pressure will cause additional hot gas to bedirected to this location, further compromising the cell at this andadjoining locations.

It should be noted that when a cell undergoes thermal runaway and ventsin a controlled fashion using the intended venting element, the cellwall may still perforate due to the size of the vent, the materialcharacteristics of the cell wall, and the flow of hot gas travelingalong the cell wall as it rushes towards the ruptured vent. Once thecell wall is compromised, i.e., perforated, collateral damage canquickly escalate, due both to the unpredictable location of such a hotspot and due to the unpredictable manner in which such cell wallperforations grow and affect neighboring cells. For example, if the cellis one of a large array of cells comprising a battery pack, the jet ofhot gas escaping the cell perforation may heat the adjacent cell toabove its critical temperature, causing the adjacent cell to enter intothermal runaway. Accordingly, it will be appreciated that theperforation of the wall of one cell during thermal runaway can initiatea cascading reaction that can spread throughout the battery pack.Furthermore, even if the jet of hot gas escaping the cell perforationfrom the first cell does not initiate thermal runaway in the adjacentcell, it may still affect the health of the adjacent cell, for exampleby weakening the adjacent cell wall, thereby making the adjacent cellmore susceptible to future failure.

As previously noted, cell perforations are due to localized, transienthot spots where hot, pressurized gas from a concentrated thermal eventis flowing near the inner surface of the cell. Whether or not a celltransient hot spot perforates the cell wall or simply dissipates andleaves the cell casing intact depends on a number of factors. Thesefactors can be divided into two groups; those that are based on thecharacteristics of the thermal event and those that are based on thephysical qualities of the cell casing. Factors within the first groupinclude the size and temperature of the hot spot as well as the durationof the thermal event and the amount of gas generated by the event.Factors within the second group include the wall thickness as well asthe casing's yield strength as a function of temperature, heat capacityand thermal conductivity.

FIG. 2 illustrates one conventional approach to improving the failureresistance of a cell, where failure is defined as a thermally inducedwall perforation. As shown, in cell 200 the thickness of casing 201 hasbeen significantly increased, thereby improving the cell's failureresistance at the expense of cell weight.

U.S. Pat. No. 6,127,064 discloses an alternate approach to the design ofa cell casing. This patent proposes to provide a lightweight, highrigidity cell casing that can suppress case deformation, the disclosedcell casing being formed by deep-drawing a clad material. Preferably theclad material is formed by diffusion-bonding an aluminum sheet and aniron sheet together, the aluminum sheet providing low weight and theiron sheet providing high rigidity. In at least one embodiment, the cladmaterial used during the deep-drawing fabrication step also includes alayer of nickel on the inner surface of the iron sheet and a secondlayer of nickel on the outer surface of the iron sheet and interposedbetween the iron and aluminum sheets, the nickel layers providingcorrosion resistance.

While the techniques described above and known in the prior art may beused to achieve high strength battery casings, in general thesetechniques require substantial wall thicknesses, and thus undue weight,in order to achieve the desired wall strength. Additionally, althoughthese techniques improve wall strength and rigidity, they do notnecessarily improve the thermal behavior of the wall during a thermalevent (e.g., thermal runaway). Accordingly, what is needed is a celldesign that can help maintain cell wall integrity during a thermal eventthrough a combination of high strength and improved thermal behavior.The present invention provides such a cell design

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for inhibiting theflow of hot, pressurized gas from within a battery through perforationsformed in the battery casing during a thermal runaway event.

In at least one embodiment of the invention, a battery assembly isprovided comprised of a battery and a pre-formed sleeve positionedaround the pre-formed battery cell case, the pre-formed sleeveinhibiting the escape of hot, pressurized gas from within the batterythrough a perforation formed in the outer surface of the battery cellcase during a thermal runaway event. The pre-formed sleeve may becomprised of a single layer of a high yield strength material; may becomprised of a single layer of a material with a yield strength of atleast 250 MPa; may be comprised of a single layer of a material with ayield strength of at least 250 MPa at a temperature of 1000° C.; and/ormay be comprised of a single layer of a material with a yield strengthof at least 500 MPa. The pre-formed sleeve may be comprised of aplurality of layers, wherein each of the plurality of layers iscomprised of a high yield strength material; wherein each of theplurality of layers is comprised of a material with a yield strength ofat least 250 MPa; wherein each of the plurality of layers is comprisedof a material with a yield strength of at least 250 MPa at a temperatureof 1000° C.; and/or wherein each of the plurality of layers is comprisedof a material with a yield strength of at least 500 MPa. The pre-formedsleeve may be comprised of a plurality of layers of at least twodifferent high yield strength materials, wherein each of the at leasttwo different high yield strength materials has a yield strength of atleast 250 MPa; wherein each of the at least two different high yieldstrength materials has a yield strength of at least 250 MPa at atemperature of 1000° C.; and/or wherein each of the at least twodifferent high yield strength materials has a yield strength of at least500 MPa. Exemplary high yield strength materials include engineeringsteels, high strength structural steels, plated steel, stainless steel,titanium, titanium alloys, and nickel alloys.

In at least one embodiment of the invention, a battery assembly isprovided comprised of a battery and a pre-formed secondary canpositioned around the pre-formed battery cell case, the pre-formedsecondary can inhibiting the escape of hot, pressurized gas from withinthe battery through a perforation formed in the outer surface of thebattery cell case during a thermal runaway event. The pre-formedsecondary can may be comprised of a single layer of a high yieldstrength material; may be comprised of a single layer of a material witha yield strength of at least 250 MPa; may be comprised of a single layerof a material with a yield strength of at least 250 MPa at a temperatureof 1000° C.; and/or may be comprised of a single layer of a materialwith a yield strength of at least 500 MPa. The pre-formed secondary canmay be comprised of a plurality of layers, wherein each of the pluralityof layers is comprised of a high yield strength material; wherein eachof the plurality of layers is comprised of a material with a yieldstrength of at least 250 MPa; wherein each of the plurality of layers iscomprised of a material with a yield strength of at least 250 MPa at atemperature of 1000° C.; and/or wherein each of the plurality of layersis comprised of a material with a yield strength of at least 500 MPa.The pre-formed secondary can may be comprised of a plurality of layersof at least two different high yield strength materials, wherein each ofthe at least two different high yield strength materials has a yieldstrength of at least 250 MPa; wherein each of the at least two differenthigh yield strength materials has a yield strength of at least 250 MPaat a temperature of 1000° C.; and/or wherein each of the at least twodifferent high yield strength materials has a yield strength of at least500 MPa. Exemplary high yield strength materials include engineeringsteels, high strength structural steels, plated steel, stainless steel,titanium, titanium alloys, and nickel alloys.

In at least one embodiment of the invention, a method of inhibiting theflow of hot, pressurized gas from within a battery through a perforationformed in an outer surface of the battery case during a thermal runawayevent is provided, the method comprising the step of fitting apre-formed sleeve around the outer surface of the battery case afterformation of the battery case, the method further comprising the step ofselecting a high yield strength material for the pre-formed sleeve,where the high yield strength material has a yield strength of at least250 MPa, and where the fitting step is performed after formation of thebattery case and prior to the thermal runaway event. Further, thefitting step may be performed after assembly of the battery.

In at least one embodiment of the invention, a method of inhibiting theflow of hot, pressurized gas from within a battery through a perforationformed in an outer surface of the battery case during a thermal runawayevent is provided, the method comprising the step of fitting apre-formed secondary can around the outer surface of the battery caseafter formation of the battery case, the method further comprising thestep of selecting a high yield strength material for the pre-formedsecondary can, where the high yield strength material has a yieldstrength of at least 250 MPa, and where the fitting step is performedafter formation of the battery case and prior to the thermal runawayevent. Further, the fitting step may be performed after assembly of thebattery.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional illustration of a cell utilizingthe 18650 form-factor;

FIG. 2 illustrates the cell shown in FIG. 1, modified to increasefailure resistance in accordance with a prior art approach;

FIG. 3 illustrates a preferred embodiment of the invention utilizing ahigh yield strength sleeve;

FIG. 4 illustrates a preferred embodiment of the invention utilizing ahigh yield strength secondary can;

FIG. 5 illustrates a modification of the embodiment shown in FIG. 3, themodified configuration utilizing a multi-layer sleeve;

FIG. 6 illustrates a modification of the embodiment shown in FIG. 4, themodified configuration utilizing a multi-layer secondary can;

FIG. 7 illustrates a modification of the embodiment shown in FIG. 3, themodified configuration including upper and lower electrical contactregions; and

FIG. 8 illustrates a modification of the embodiment shown in FIG. 4, themodified configuration including an upper electrical contact region.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

In the following text, the terms “battery”, “cell”, and “battery cell”may be used interchangeably and may refer to any of a variety ofdifferent cell chemistries and configurations including, but not limitedto, lithium ion (e.g., lithium iron phosphate, lithium cobalt oxide,other lithium metal oxides, etc.), lithium ion polymer, nickel metalhydride, nickel cadmium, nickel hydrogen, nickel zinc, silver zinc, orother battery type/configuration. The term “battery pack” as used hereinrefers to multiple individual batteries contained within a single pieceor multi-piece housing, the individual batteries electricallyinterconnected to achieve the desired voltage and capacity for aparticular application. The terms “assembled battery” and “assembledcell” may be used interchangeably and refer to a cell that has completedthe fabrication and assembly steps to form an electrically functioningcell. The terms “fabricated cell casing” and “formed cell casing” may beused interchangeably and refer to a cell casing that has completed theformation/fabrication steps and is ready for inclusion of the innerbattery components (e.g., electrodes, electrolyte, spacers, capassembly, etc.). It should be understood that identical element symbolsused on multiple figures refer to the same component, or components ofequal functionality. Additionally, the accompanying figures are onlymeant to illustrate, not limit, the scope of the invention and shouldnot be considered to be to scale.

Throughout the following specification, the invention is describedrelative to cells using the 18650 form-factor. It should be understood,however, that the invention may also be applied to other cell designs,shapes, chemistries, form-factors and configurations while retaining thefunctionality of the present invention. For example, the application isequally applicable to prismatic and pouch cells.

The present inventors recognize the weight constraints placed on thebatteries within a large battery pack and the factors that contribute tothe initiation and growth of wall perforations during thermal runaway.Additionally, the present inventors recognize that once a cell entersinto thermal runaway it is no longer viable, and therefore at this pointthe primary purpose of the cell casing is to control the direction andpathway for the hot, escaping gas generated by the thermal runawayevent. In recognition of these design parameters, the intent of thepresent invention is to minimize, if not altogether eliminate, theescape of hot, pressurized gas from the sides of a cell where theescaping gas can adversely affect neighboring cells. Rather than allowthe hot gas to escape through wall perforations, the present inventionforces this gas to exit the cell from either end surface, or in someembodiments, from only one of the cell end surfaces.

FIG. 3 illustrates a first preferred embodiment of the invention. Inthis embodiment, a conventional cell such as that shown in FIG. 1 ismodified by positioning a pre-formed sleeve 301 lengthwise over theouter casing wall 101. Preferably pre-formed sleeve 301 is added afterassembly of the cell has been completed, but prior to the inclusion ofthe cell within the cell's intended application, e.g., a battery pack.Alternately, pre-formed sleeve 301 may be added after fabrication of thecell casing and in a separate fabrication step, but prior to cellassembly. As shown, in this embodiment sleeve 301 only covers thesidewalls 303 of the cell case, not the bottom surface 305 of thecasing. FIG. 4 illustrates a modification of this embodiment in which apre-formed secondary can 401 is positioned lengthwise over the cellcasing, thereby covering both sidewalls 303 and bottom cell surface 305.As in the embodiment illustrated in FIG. 3, pre-formed secondary can 401is either added after the cell has been assembled, or after theformation/fabrication of the cell casing 101 but prior to batteryassembly.

Regardless of whether a sleeve is added to the cell as shown in FIG. 3,or a secondary can is added to the cell has shown in FIG. 4, preferablythere is minimal clearance between the inner surface of sleeve 301 orinner surfaces of the can 401 and the outer surface(s) of the cellcasing. In at least one embodiment, the sleeve or secondary can ispress-fit or force-fit onto the cell casing, for example using ahydraulic press-fit system.

The inventors have found that during a thermal runaway event, thebehavior of a cell with a sleeve or secondary can as shown in FIGS. 3and 4 is quite different from a conventional cell, even if theconventional cell is fabricated from a material that has the samecomposition and thickness as a cell fabricated in accordance with theinvention. The differences in behavior are largely due to the thermalcontact resistance that occurs between the sleeve and cell casing of theassembled cell, or the secondary can and the cell casing of theassembled cell. For example, the thermal resistance of an 18650 cellwith a single stainless steel wall of thickness 0.13 millimeters is8.156×10⁻⁶ C m²/W. In contrast, the thermal resistance of an 18650 cellwith a dual stainless steel wall in which the two walls have a combinedthickness of 0.13 millimeters and in which the outer wall is in the formof a pre-formed sleeve added after cell assembly is 2.082×10⁻⁴ C m²/W.Accordingly, with all other factors being the same, adding a sleeveincreases the thermal resistance by a factor of approximately 25. As aresult, for this configuration a cell with an internal temperature of1700 K will transfer only about 60 watts of thermal energy viaconduction in contrast to over 1500 watts for the non-sleeved,conventional configuration.

The conventional wisdom regarding the design of battery casings has beento change the composition and/or thickness of the casing during casefabrication in order to achieve the desired cell properties, propertiessuch as wall strength, corrosion resistance, lower weight, etc. Whilethese goals may be achieved through case design, the present inventorshave found that these improvements do not adequately achieve the goalsof the present invention, i.e., increasing the resistance of the cellcasing to perforation during a thermal event as well as increasing theresistance to propagation of the thermal event to adjacent cells.

The use of a separate sleeve/secondary can offers a number of advantagesover the conventional approach of simply increasing the wall thicknessof casing 101 and/or fabricating the wall from multiple layers ofvarious materials. First and foremost, the goals of significantlydecreasing the risk of hot, pressurized gas escaping through the cellwall and redirecting this escaping gas to the cell ends are bothachieved while adding much less weight to the cell than would berequired to achieve the same performance using the conventionalapproach. Second, as sleeve 301 (or can 401) is added after completionof cell assembly, or at least added after cell case fabrication, thisapproach can be used with virtually any manufacturer's cell since itdoes not affect the cell manufacturing process. Third, while thematerial selected for casing 101 must be non-reactive with the cellcontents (e.g., electrolyte and electrode assembly), no such materialconstraints are placed on sleeve 301 or can 401. Accordingly thematerial used for sleeve 301, or can 401, can be selected based on itsability to minimize or eliminate the escape of hot, pressurized gas fromthe cell sidewalls.

Preferably the failure resistance of sleeve 301, or can 401, is furtherenhanced by fabricating the sleeve from a material that exhibits highyield strength at high temperature. Preferably the yield strength ofsleeve 301, or can 401, is at least 250 MPa at room temperature, morepreferably at least 250 MPa at a temperature of 1000° C., still morepreferably at least 500 MPa at room temperature, and yet still morepreferably at least 500 MPa at a temperature of 1000° C.

Given the thermal contact resistance between the case and the sleeve orcan as noted above, and given that the sleeve/can is preferablyconstructed of a high yield strength material, typically only casing 101will be perforated during thermal runaway, leaving outer sleeve 301 (orcan 401) intact. As a result, adjacent cells are not subjected to astream of high temperature, pressurized gas. Additionally, in thissituation, and given the minimal clearance between the outer surface ofcase 101 and the inner surface of either sleeve 301 or can 401, sleeve301 or can 401 will prevent the rapid growth of the case wallperforation, thereby minimizing the amount of gas that escapes throughthe case wall perforation. This effect is aided by the cell casedimensions expanding slightly during thermal runaway due to theincreased internal pressure, thereby further improving the contactbetween the outer cell surface and the interior surface of the sleeve orcan. Accordingly little, if any, hot gas escapes through the case wallperforation and that which does escape is redirected by sleeve 301 toeither cell end, or by can 401 to the upper cell end, where its effectson neighboring cells are minimized. Preventing the flow of hightemperature, pressurized gas from the affected cell combined with thesubstantially decreased heat conduction from the sleeved cell leads tosubstantial reduction in thermal energy being transferred from the cellundergoing thermal runaway to adjacent cells, thereby decreasing thechances of a single thermal event propagating throughout the batterypack.

It will be appreciated that there are a variety of materials suitablefor use in constructing sleeve 301 or can 401. Exemplary materialsinclude various engineering and high strength structural steels, platedsteel, stainless steel, titanium and titanium alloys, and nickel alloys.The preferred technique or techniques for fabricating sleeve 301/can 401depend on the selected material as well as the shape of the cell forwhich the sleeve or can is to be used. For example, if the cell has the18650 form-factor, the sleeve/can is cylindrically shaped. Sleeve 301and can 401 may be fabricated using any of a variety of techniquesincluding, but not limited to, (i) drawing the sleeve, (ii) wrapping astrip or sheet of the desired material around the cell and welding ormechanically coupling the edges together, (iii) bending and welding astrip or sheet of the desired material into the desired shape, or (iv)bending a strip or sheet of the desired material into the desired shapeand coupling the two edges together using mechanical interconnects. Thesleeve or can may be bonded into place.

FIGS. 5 and 6 illustrate a modification of the embodiments shown inFIGS. 3 and 4, respectively, in which the sleeve 501 or can 601 isfabricated from a plurality of layers 503-505. Preferably three layersare used, as shown, although it will be appreciated that either a fewernumber or a greater number of layers may be used in these embodiments.Preferably all of the layers are comprised of a material exhibiting highyield strength at high temperatures, more specifically a yield strengthof at least 250 MPa at room temperature, still more preferably at least250 MPa at a temperature of 1000° C., still more preferably at least 500MPa at room temperature, and yet still more preferably at least 500 MPaat a temperature of 1000° C. The plurality of layers may be comprised ofthe same material, or of one or more different materials. Exemplarymaterials include various engineering and high strength structuralsteels, plated steel, stainless steel, titanium and titanium alloys, andnickel alloys.

In the embodiments illustrated in FIGS. 3 and 5 that utilize a sleeve,typically electrical contact is made to the cell casing, assuming thatthe cell casing is one of the cell's terminals as in an 18650form-factor cell, at the bottom, exposed surface 305. Alternately,assuming that the material(s) comprising the sleeve is electricallyconductive and that there is no electrical insulator (e.g., a bondingmaterial) interposed between the outer cell sidewall and the sleeve,contact may be made through the sleeve. Alternately, and as illustratedin the exemplary embodiment shown in FIG. 7, an upper region 701 and/ora lower region 703 of cell sidewall 303 may be left uncovered by sleeve301 (or sleeve 501), thus allowing electrical contact to be made to thecell via this region or regions. Preferably region 701 and/or region 703is in the form of a ring that extends around the circumference of thecell case as shown with a width, measured from the respective end of thecell casing, of less than 5 millimeters, more preferably between 2 and 4millimeters, and still more preferably between 2 and 3 millimeters.

In the embodiments illustrated in FIGS. 4 and 6 that utilize a secondarycan, electrical contact may be made to the cell casing through thesecondary can, assuming that the material(s) comprising the secondarycan is electrically conductive and that there is no electrical insulator(e.g., a bonding material) interposed between the outer cell casing andthe inside surface of the can. Alternately, and as illustrated in theexemplary embodiment shown in FIG. 8, an upper region 801 of cellsidewall 303 is left uncovered by the can, thus allowing electricalcontact to be made to the cell via this region. Preferably region 801 isin the form of a ring that extends around the circumference of the cellcase as shown with a width, measured from the top of the cell casing, ofless than 5 millimeters, more preferably between 2 and 4 millimeters,and still more preferably between 2 and 3 millimeters. Preferably region801 is located above crimp 803 as the region above crimp 803 is lesssusceptible to the formation of hot spots as it lies above electrodeassembly 103.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

1. A battery assembly, comprising: a battery, said battery comprising: apre-formed cell case having an exterior sidewall surface, a first endportion and a second end portion, wherein said first end portion iscomprised of a cell case bottom, and wherein said second end portion iscomprised of a cap assembly retention lip and a central open portion; anelectrode assembly contained within said pre-formed cell case, wherein afirst electrode of said electrode assembly is electrically connected tosaid pre-formed cell case; and a cap assembly mounted to said pre-formedcell case, said cap assembly closing said central open portion of saidsecond end portion, wherein said cap assembly further comprises abattery terminal electrically isolated from said pre-formed cell caseand electrically connected to a second electrode of said electrodeassembly; and a pre-formed sleeve positioned around and in contact withsaid pre-formed cell case, wherein said pre-formed sleeve is formedseparately from said pre-formed cell case, wherein an interior surfaceof said pre-formed sleeve is proximate to said exterior sidewall surfaceof said pre-formed cell case, wherein a case contact region of saidexterior sidewall surface of said pre-formed cell case remains uncoveredby said pre-formed sleeve, wherein said case contact region is proximateto an end surface of said pre-formed cell case, wherein said pre-formedsleeve is comprised of a high yield strength material, wherein saidpre-formed sleeve inhibits the escape of hot, pressurized gas fromwithin said battery through said exterior sidewall surface of said cellcase during a thermal runaway event, and wherein said pre-formed sleevedirects the flow of said hot, pressurized gas towards said first endportion or said second end portion.
 2. The battery assembly of claim 1,wherein said battery is pre-assembled prior to positioning saidpre-formed sleeve around said pre-formed cell case.
 3. The batteryassembly of claim 1, wherein said pre-formed sleeve is press-fit aroundsaid pre-formed cell case.
 4. The battery assembly of claim 1, whereinsaid battery has an 18650 form-factor.
 5. The battery assembly of claim1, wherein said high yield strength material is selected from the groupof materials consisting of steels, plated steel, stainless steel,titanium, titanium alloys, and nickel alloys.
 6. The battery assembly ofclaim 1, wherein said pre-formed sleeve is comprised of a single layerof said high yield strength material.
 7. The battery assembly of claim6, wherein said high yield strength material has a yield strength of atleast 250 MPa.
 8. The battery assembly of claim 6, wherein said highyield strength material has a yield strength of at least 250 MPa at atemperature of 1000° C.
 9. The battery assembly of claim 6, wherein saidhigh yield strength material has a yield strength of at least 500 MPa.10. The battery assembly of claim 1, wherein said pre-formed sleeve iscomprised of a plurality of layers, wherein each of said plurality oflayers is comprised of said high yield strength material.
 11. Thebattery assembly of claim 10, wherein said high yield strength materialhas a yield strength of at least 250 MPa.
 12. The battery assembly ofclaim 10, wherein said high yield strength material has a yield strengthof at least 250 MPa at a temperature of 1000° C.
 13. The batteryassembly of claim 10, wherein said high yield strength material has ayield strength of at least 500 MPa.
 14. The battery assembly of claim 1,wherein said pre-formed sleeve is comprised of a plurality of layers,wherein said plurality of layers are comprised of at least two differenthigh yield strength materials.
 15. The battery assembly of claim 14,wherein each of said at least two different high yield strengthmaterials has a yield strength of at least 250 MPa.
 16. The batteryassembly of claim 14, wherein each of said at least two different highyield strength materials has a yield strength of at least 250 MPa at atemperature of 1000° C.
 17. The battery assembly of claim 14, whereineach of said at least two different high yield strength materials has ayield strength of at least 500 MPa.
 18. The battery assembly of claim 1,wherein said case contact region is proximate to said end surface ofsaid second end portion of said pre-formed cell case.
 19. The batteryassembly of claim 1, wherein said case contact region is proximate tosaid end surface of said first end portion of said pre-formed cell case.